Illustrated herein in embodiments are imaging members, such as electrostatographic imaging members exhibiting enhanced service life.
Electrostatographic imaging members are known in the art. Typical electrostatographic imaging members include, for example, photoreceptors for electrophotographic imaging systems and electroreceptors such as ionographic imaging members for electrographic imaging systems. Generally, these imaging members comprise at least a supporting substrate and at least one imaging layer comprising a thermoplastic polymeric matrix material. In a photoreceptor, the photoconductive imaging layer may comprise only a single photoconductive layer or a plurality of layers such as a combination of a charge generating layer and one or more charge transport layer(s).
Electrostatographic imaging members can be in the form of a number of different configurations. For example, they can comprise a flexible member formed by utilizing a flexible supporting substrate layer, or a rigid member, such as a drum. In this regard, flexible imaging members may consist of a flexible scroll configuration or a belt which may be seamed or seamless. Drum imaging members have a rigid cylindrical supporting substrate bearing one or more imaging layers.
The flexible electrophotographic imaging member belts are typically fabricated from a sheet which is cut from a web. The sheets are generally rectangular in shape. The edges may be of the same length or one pair of parallel edges may be longer than the other pair of parallel edges. The sheets are formed into a belt by joining overlapping opposite marginal end regions of the sheet. A seam is typically produced in the overlapping marginal end regions at the point of joining. Joining may be effected by any suitable means. Typical joining techniques include welding (including ultrasonic), gluing, taping, pressure heat fusing, and the like. Ultrasonic welding is generally the more desirable method of joining because it is rapid, clean (no solvents) and produces a thin and narrow seam. In addition, ultrasonic welding is more desirable because it causes generation of heat at the contiguous overlapping end marginal regions of the sheet to maximize melting of one or more layers therein to produce a strong fusion bonded seam.
A typical flexible electrophotographic imaging member belt comprises at least one photoconductive insulating layer. It is imaged by uniformly depositing an electrostatic charge on the imaging surface of the electrophotographic imaging member and then exposing the imaging member to a pattern of activating electromagnetic radiation, such as, light which selectively dissipates the charge in the illuminated areas of the imaging member while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking toner particles on the imaging member surface. The resulting visible toner image can then be transferred to a suitable receiving member or substrate such as paper.
A number of current flexible electrophotographic imaging members are multilayered photoreceptors that, in a negative charging system, comprise a substrate support, an electrically conductive layer, an optional charge blocking layer, an optional adhesive layer, a charge generating layer, and a charge transport layer. In such an imaging member, the charge transport layer is the top outermost layer exposed to the environment. An anti-curl layer may, for example, also be employed on the back side of the flexible substrate support, the side opposite to the electrically active layers, to achieve the desired photoreceptor belt flatness.
When a negatively charged photoreceptor belt is functioning under normal machine conditions of image creation and processing, the belt is mounted over and around a belt support module. As such, the belt is constantly subjected to bending strain as it flexes over each of the belt module support rollers during dynamic belt cyclic motion. The greatest bending strain is tension concentrated at the surface of the charge transport layer, so that extended belt cyclic flexing has been found to facilitate the development of surface cracking.
In this regard, surface cracking in the charge transport layer is somewhat unique only in belt photoreceptors and is induced, in part, due to the effect of dynamic fatigue of the belt flexing over the supporting rollers of a machine belt support module. Furthermore, surface cracking has also been found to be caused by exposure to airborne chemical contaminants as the photoreceptor segments statically “park” or directly bend over the rollers after periods of photoreceptor belt non-use during machine idling. Typical chemical contaminants that a photoreceptor belt can be exposed to include solvent vapors, environment airborne pollutants, and corona species emitted by machine charging subsystems. Surface cracking can also be exacerbated by the combination of the effects provided by fatigue belt flexing and airborne chemical exposure. In fact, the problem of photoreceptor surface cracking is a critical mechanical issue seen in imaging members, particularly, in flexible belts, because the cracks manifest themselves into printout defects that seriously impact copy quality. Similarly, the charge transport layer has also been found to be susceptible to surface scratching which often produces copy defect problems as well.
Furthermore, each charge transport layer of multi-layered photoreceptors is typically formed by a solution coating processes. The coating solutions generally contain from about 75% wt to about 91% wt organic solvent(s), such as methylene chloride or a chlorinated solvent. After application of the coating solution, the wet coating layer is dried at elevated temperatures to remove a substantial amount of the solvent to produce a solid layer.
Since solutions used by conventional coating processes are prepared utilizing organic solvents, it has been found that not all of the solvent may be removed from the coating layer during drying. For example, during the production solution coating of a typical charge transport layer containing about 86% wt methylene chloride solvent and 14% wt dissolve solid, the solvent evaporates very quickly during the elevated temperature drying process. However, about 2% wt of the methylene chloride will typically still be present or trapped in the resulting charge transport layer (i.e., residual methylene chloride). The trapped solvent may evaporate or “outgas” over time. However, the eventual out gassing of the trapped solvent from the charge transport layer after storage and over the life of the photoreceptor causes dimensional contraction of the charge transport layer. This results in a build-up of internal strain in the charge transport layer. Thus, in addition to the bending strain induced during dynamic photoreceptor belt flexing over each belt module support roller in a machine, this build-up of internal strain will exacerbate charge transport layer cracking under normal belt functioning conditions in the field.
Furthermore, dimension contraction in the charge transport layer causes the photoreceptor belt to exhibit upward curling at both edges when the belt functions in a machine. Since the contraction in belt direction is prevented by the applied tension as the belt is mounted over and around a belt support module, exhibition of edge curling in the photoreceptor belt is an important issue. This is because, in part, edge curling changes the distance between the belt surface to the charging device, causing non-uniform surface charging density which is then reflected as a “smile print” defect. Such a defect is characterized by higher intensity print-images at the locations over both belt edges.
Moreover, while much of the solvent vapor emission produced during the solution coating applications can be recovered by various abatement processes to prevent release of the solvent vapor into the atmosphere, these processes are costly and not fully efficient. Hence, a need also exists for the formation of a charge transport layer such that its coating does not create significant environmental pollutant emissions or cause safety and health issues.
Furthermore, since the charge transport layer of a typical negatively charged multilayered photoreceptor belt is the top outermost exposed layer, such a charge transport layer is inevitably subjected to constant mechanical interactions against various electrophotographic imaging machine subsystems under a normal service environment. These interactions include abrasive contact with cleaning and/or spot blades, exposure to toner particles, carrier beads, toner image receiving substrates, etc. Therefore, the charge transport layer may frequently exhibit mechanical failures such as frictional abrasion, wear, and surface cracking due to fatigue dynamic belt flexing. This can also be exacerbated by solvent vapor exposure, etc. Accordingly, a further need exists to provide protective coverage over the charge transport layer to effectively surpass these mechanical difficulties.
To resolve one or more of the above-noted shortcomings and issues, various methods of fabrication of improved electrophotographic imaging members have been investigated and successfully demonstrated as noted below. The imaging members produced thereby exhibit good cracking resistance, wear resistance, and durability. Such imaging member belts exhibit enhanced physical/mechanical functioning life and a reduced charge transport layer cracking, etc.